This application claims priority on Provisional Appl. No. 60/015,926, filed Apr. 22, 1996.
1. Field of the Invention
The subject invention relates to a metal gasket for a fitting in a high purity gas line.
2. Description of the Prior Art
Gaseous fluids are used in many manufacturing processees, including the manufacture of microprocessors and other electric components. Gases are also used in various chemical engineering processees, in medical applications and in machining or welding operations. The required gas may vary from one industrial application to the next.
Purity of the specified gas often is critical to the manufacturing process. Thus, it is important to prevent the diffusion or leakage of atmospheric contaminants through the gas delivery system and into the gas stream.
A prior art fitting for high pressure fluid applications is shown in FIG. 1 and is identified generally by the numeral 10. This prior art fitting 10 includes a first component 12 with a generally tubular wall 14 defining a fluid passage 16 extending axially therethrough. The first component further includes an enlarged axial end 18 having a shoulder 20 facing the tubular wall 14 of the first component and an opposed sealing face 22. A toroidally generated bead 24 is unitarily defined on the sealing face 22 and is concentric with the fluid passage 16 through the first component 12.
The prior art fitting further includes a second component 32 with a tubular wall 34 and a fluid passage 36 extending axially therethrough. The second component 32 includes an axial end 38 with an annular shoulder 40 facing the tubular wall 34 of the second component 32 and an opposed sealing face 42. The sealing face 42 of the second component 32 includes a toroidally generated bead 44 dimensioned and configured for registration with the toroidal bead 24 on the sealing face 32 of the first component 12.
The prior art fitting further includes male and female coupling nuts 46 and 48 having portions dimensioned and disposed for engaging the respective shoulders 20 and 40 of the first and second components 12 and 32 respectively. The threads of the nuts 46 and 48 are dimensioned and pitched for threaded engagement with one another. The threaded tightening of the nuts urges the sealing faces 22 and 42 of the components 12 and 32 toward one another.
The prior art fitting 10 further includes an annular metal gasket 50 disposed between the first and second components 12 and 32 respectively. More particularly, the prior art gasket 50 includes a first sealing face 52 facing the sealing face 22 of the first component 12, a second sealing face 54 facing the sealing face 42 of the second component 32 and a central passage 55 extending therebetween. Tightening of the nuts 46 and 48 urges the toroidal sealing beads 24 and 44 of the first and second components 12 and 32 respectively into tight sealing engagement with the opposed faces 52 and 54 of the gasket 50.
Some prior art gaskets have planar parallel sealing faces that engage with the toroidal sealing beads 24 and 44. However, the prior art gasket 50 shown in FIG. 1 is provided with annular sealing grooves 56 and 58 formed on the respective first and sealing faces 52 and 54. The grooves 56 and 58 are dimensioned to receive the toroidal sealing beads 24 and 44 therein.
The prior art grooved gasket as shown in FIG. 1 offers several advantages over the prior art gasket with opposed planar sealing faces. For example, the grooves 56 and 58 on the seal faces 52 and 54 on the gasket 50 are recessed in such a way that they are less likely to be damaged by dents or scratches generated during the production, transport, and handling of the gaskets.
The grooved gasket also has provided more than one location at which a pressure tight seal may be established between the toroidal sealing beads 22 and 42 of the components 12 and 32 and the recessed grooves 56 and 58 of the gasket 50. In particular, the toroidal sealing beads 24 and 44 initially contact radially outer portions 64 and 74 of the grooves 56 and 58. However after sufficient tightening, the toroidal sealing beads 24 and 44 may slide into sealing contact with other surfaces of the prior art gasket 50. This reduces the chance that a localized defect on either the gasket 50 or the toroidal sealing bead 24 or 44 will result in a joint leak. In particular, even if a defect extends across one of the circular bands of contact between the gasket 50 and the toroidal sealing bead 24 or 44, the other circular band of contact may remain uncompromised.
Another advantage of the prior art groove gasket can be explained more clearly with reference to FIG. 2. In particular, the prior art toroidal sealing beads 24 and 44 have been dimensioned to engage a radially outer sloped surface 64, 74 leading into the respective grooves 56 and 58. A given amount of axial force applied to the components as the joint is tightened will result in a sealing load, designated as force per inch of seal circumference, that is higher than the seal load which results when a gasket surface that is normal to the axis of the joint contacts a toroidal sealing bead. This increased sealing load makes a tight joint easier to achieve and/or more likely to occur if the parts are tightened to a constant torque. Specifically, a higher sealing force will produce more gasket deformation. If the nut used to tighten the joint is rotated through a constant angle, less torque will be required to achieve this angle of rotation.
With a flat gasket, there is almost no radial motion or relative sliding of the gasket and toroidal sealing bead contact circle as the joint is tightened. However, the prior art alignment of the toroidal sealing bead 24, 44 with radially outer portions 64, 74 of the grooves 56, 58 causes the gasket 50 and the toroidal sealing beads 24, 44 to slide over each other as the sealing load increases. If a soft metallic coating, such as silver plating, is applied to the gasket faces, the sliding contact between the seal surfaces is likely to push a displaced surface layer away from the area of highest contact pressure. This results in the filling of minor scratches, pits and other depressions which might otherwise cause a leaky joint by bridging the narrow zone of contact characteristic of toroidal all-metal base seal joints.
Another advantage of prior art grooved gasket 50 relates to the ability of the gasket 50 to aid in locating the central aperture 55 concentrically to each of the toroidal sealing beads 24 and 44 with which the gasket 50 mates and in locating the components 12 and 32 having the toroidal sealing beads 24 and 44 concentrically with one another. This is helpful in avoiding a partial misalignment of the gasket 50 into an eccentric position where the gasket 50 partly blocks the through passages 16 and 36 through the components 12 and 32.
Finally, the prior art grooved gasket 50 localizes the areas on the gasket 50 at which contact with toroidal sealing beads 24 and 44 may be made. As a result, the areas of the gasket that must be inspected for defects is substantially reduced, thereby facilitating the inspection process.
A disadvantage of the prior art grooved gaskets has been the creation of a dead zone that is substantially isolated from the main flow path as indicated by the cross-hatching in FIG. 3. This creates a situation detrimental to the achievement and maintenance of conditions required for chemical and manufacturing processees that are dependent on high levels of purity, low levels of moisture, freedom from crevice corrosion and an ability to be sterilized or decontaminated.
Additionally, the prior art grooved gasket 50 shown in FIGS. 2 and 3 results in the toroidal sealing beads 24, 44 sliding along the sloped radially outer surfaces 64, 70 of the grooves 56, 58 as the coupling is tightened. This sliding generates debris in the form of small particles of metal. These small particles of metal will be at the axial extreme of the sliding movement, which, in the prior art shown in FIGS. 2 and 3 also is at the radially inner most region of sliding contact. Metal debris separated as the fitting is tightened and as the surfaces slide relative to one another can work their work through the cross-hatched dead area of FIG. 3 and into the gas stream. These metallic components are contaminants that may be unacceptable in many high purity installations, such as installations used in the semi-conductor industry.